The present invention relates generally to accelerometers, and more specifically to techniques for improving the manufacturing test and calibration throughput of integrated convective accelerometers.
U.S. Pat. No. 6,795,752 entitled THERMAL CONVECTION ACCELEROMETER WITH CLOSED-LOOP HEATER CONTROL issued Sep. 21, 2004 and assigned to the same assignee of the present invention (the “'752 patent”) discloses an integrated convective accelerometer comprising a convective acceleration sensor that includes a pair of temperature sensing elements disposed on opposing sides of a heater element. When an acceleration is applied to the integrated convective accelerometer along a sense axis passing through the heater element and the pair of temperature sensing elements within the convective acceleration sensor, the sensor produces a differential output voltage proportional to the magnitude of the applied acceleration. In recent years, accelerometers like the integrated convective accelerometer disclosed in the '752 patent have been increasingly employed in high accuracy applications for sensing acceleration in a variety of environments, including environments in which temperatures can vary over a broad range. However, the outputs produced by such accelerometers can vary over temperature, thereby reducing the overall accuracy of the accelerometers.
For this reason, compensation and calibration circuitry have been employed in conjunction with accelerometers to compensate for temperature fluctuations occurring within the local environment of the accelerometer device, and to calibrate the accelerometer output gain and offset. Such compensation and calibration circuitry may be included in the accelerometer itself, or may be implemented external to the accelerometer device. For example, the integrated convective accelerometer disclosed in the '752 patent includes temperature compensation circuitry that allows the sensitivity of the convective acceleration sensor to be maintained at a desired level as the temperature of the local environment changes. Specifically, the integrated convective accelerometer includes amplification circuitry operative to extract an average output voltage from the differential output voltage produced by the convective acceleration sensor. The average output voltage provides a measure of the temperature gradient produced by the heater element within the convective acceleration sensor. The integrated convective accelerometer further includes control circuitry operative to produce a control output for regulating the average output voltage, thereby regulating the temperature gradient of the heater element. In this way, the sensitivity of the convective acceleration sensor can be maintained at the desired level over temperature.
In addition, the integrated convective accelerometer disclosed in the '752 patent includes calibration adjustment circuitry for setting the common-mode voltage drop across the temperature sensing elements within the convective acceleration sensor, and for setting the accelerometer output gain and offset. The integrated convective accelerometer is typically calibrated by adjusting one or more parameters including the offset temperature coefficient (OTC) of the accelerometer device. Specifically, in a typical calibration routine, the integrated convective accelerometer is brought to an initial temperature, e.g., 25°±2° C., and, in the absence of an applied acceleration, the output offset of the convective acceleration sensor is measured. In one calibration mode, the output offset of the convective acceleration sensor is measured for a number of settings of the common-mode voltage drop across the temperature sensing elements. Next, the accelerometer device is brought to a second higher temperature, e.g., 85°±20° C., and the output offset of the convective acceleration sensor is measured again in the absence of an applied acceleration for the same settings of the common-mode voltage drop across the temperature sensing elements. The OTC of the accelerometer device is then calculated using the output offset measurements performed at the two temperatures of the device, and an optimal setting for calibrating the accelerometer output offset is determined based upon the calculated OTC. Finally, the integrated convective accelerometer is brought back to its initial temperature, and the output offset calibration setting is programmed into the device, for example, by setting one or more internal fuses.
One drawback of the above-described technique for calibrating the output offset of an integrated convective accelerometer is that the time required for performing the calibration routine can be excessively long. For example, as described above, the integrated convective accelerometer of the '752 patent is typically calibrated by taking a number of measurements of the output offset of the convective acceleration sensor at an initial temperature, and then taking a number of additional measurements of the output offset at a higher second temperature of the accelerometer device. However, each of these output offset measurements can take on the order of 0.1 second to perform, thereby significantly reducing the manufacturing test throughput of the devices. The manufacturing test throughput of the convective accelerometer devices is further reduced if the devices are calibrated in a serial fashion.
It would therefore be desirable to have an improved technique for testing and calibrating integrated sensor devices such as integrated convective accelerometers. Such a test and calibration technique would improve the overall manufacturing test and calibration throughput of the sensor devices. It would also be desirable to have an improved manufacturing test technique that can be employed for calibrating integrated sensor devices having one or more device parameters that are temperature dependent.